Most of us are familiar with jellyfish and have occasionally encountered them as washed up animals as we walk along a shoreline. Others may have had the unfortunate experience of bumping into one whilst swimming and getting stung. Depending on the species these stings can be very painful, but apart from the occasional thought about how curious these animals look, few of us rarely consider these animals or indeed recognise them as living fossils, unchanged for over 600 million years.

Jellyfish belong to a phylum called Cnidaria (pronounced nigh-dare-ee-a). This phylum consists of a huge range of animals, almost all of them associated with marine environments. Cnidarians have a central mouth, surrounded by tentacles which possess stinging cells (called nematocysts). The tentacles are used to catch and disable prey which is then drawn into the central mouth and consumed. Within the fossil record, by far the most important type of Cnidarians represented are the corals. These animals may live in colonies or have a solitary habit. All corals secrete a hard, exoskeleton made up of calcium carbonate and the extant species today are known as scleratinian corals. These scleratinian corals (solitary or can be linked together sharing a colonial skeleton), evolved relatively recently appearing at around the beginning of the Mesozoic, after the great Permian extinction event. Older coral fossils found in the Palaeozoic fossil record are either rugose corals (solitary or colonial) or tabulate corals (always found in colonies). These types of corals both went extinct at the end of the Permian but fossil corals are known to date right back to Ordovician aged sediments.

The phylum Cnidaria also has soft-bodied representatives, these are the sea anemones and jellyfish, scientists believe that these type of animals are even older than the first corals. Being soft-bodied, jellyfish and sea anemones are very poorly represented in the fossil record, in fact these fossils are extremely rare and the number of fossil specimens found to date does not represent the diversity or indeed the abundance of these animals throughout the Phanerozoic eon (or indeed the Cryptozoic eon).

There are some strange, preserved ripples and hollowed out round depressions found in very finely grained, sandstone, Cambrian sediments from around the world. These are believed to be the fossilised remains of jellyfish. It is thought that many of the fossils of these animals that we have were formed when mass strandings of jellyfish occurred after a storm. Having been washed up onto a Cambrian or Ordovician shoreline, the carcases of these animals would not have been eaten by scavengers as there was no significant life on the land during these geological periods. Some of them may have been quickly buried and under exceptional conditions, preserved as impressions in the rocks.

It is thought that primitive life such as bacteria and cyanobacteria first evolved as far back as 3.5 billion years ago, but complex multi-celled animals appeared much later, sometime between 700 and 620 million years ago. Multi-celled animals are termed metazoans (pronounced met-ah-zo-ans) and their appearance in the fossil record marks the ending of the Cryptozoic (hidden life also known as the Precambrian) and the start of the Phanerozoic eon (visible life).

Certainly, the fossil evidence for the first metazoans is extremely poor. All the first complex creatures were entirely soft-bodied but their fossils are evidence of complex multi-celled organisms living in late Precambrian times. The Ediacaran Hills in the Flinders range of mountains in South Australia was one of the first places studied. This is why the fossils of such organisms are often referred to as “Ediacarans”. This was a world dominated by jelly and from fossils found in places as diverse as Siberia and Namibia, scientists are able to calculate that even 620 million years ago, primitive life was abundant and diverse. The jelly structures of the Ediacarans may have been more robust than those found in jellyfish, after all, jellyfish consist of 95% water, this may explain why Ediacaran animals may have been more readily fossilised than jellyfish. However, certainly by the Cambrian, jellyfish were very much established and as carnivores would have been perhaps one of the most feared predators around during the early Palaeozoic.

An Illustration of a Jellyfish

Picture Credit: Everything Dinosaur

The picture above is of a typical jellyfish with a bell-like structure and trailing tentacles. Jellyfish are often termed Medusa, a reference to Greek mythology and the Gorgon Medousa who had snakes instead of hair.

Animals as diverse as jellyfish and corals are grouped in the Cnidaria phylum as all these organisms share a central mouth surrounded by stinging tentacles (termed nematocysts). Scientists believe that corals, sea anemones and jellyfish are all descended from a common ancestor, that had these features. There are many thousands of species of jellyfish found today. They live in polar seas as well as the tropics and can be found right through the water column with nektonic forms (animals that live above the sea floor), being found in shallow seas and at extreme depths. It is thought that there have been hundreds of thousands of species of jellyfish, but only a few specimens have been preserved in the fossil record. Jellyfish have no definite head structure, no internal organs as such and most have only the most primitive senses. Jellyfish are very much dependent on sea currents to move them, most species drift with the prevailing current, although a number of Classes can pulse their bells to help them move vertically through the water column.

Although regarded as extremely simple animals when compared to the Chordates for example, during the late Precambrian and into the Cambrian these types of animals may have been amongst the top predators around at the time. Jellyfish predating on other organisms may have been a causal factor in the speeding up of evolution (known as the Cambrian explosion), as predator/prey relationships developed. Our ancestors, those simple animals with a notochord would have fallen victim to the stinging tentacles of the jellyfish. Perhaps they began to evolve improved senses, a significant head end to house these new senses and stronger groups of paired muscles around a central nervous strip to help them escape from their jellyfish foes.

The next time you see a jellyfish washed up on a beach as you walk along a shoreline, you might well give it a second glance. It represents a type of animal that has been around for something like 600 million years and its evolution may well have contributed to the evolution of the earliest Chordates and the speeding up of evolution as predator/prey relationships developed. Jellyfish can certainly be described as “living fossils”.